In general, epidural anesthesia has increasingly been adopted in various analgesic treatments for surgery, obstetric surgery, etc. In the epidural anesthesia, it is necessary to ascertain whether a tip of the needle enters the epidural space (cavity) in the spinal column. The epidural space is located across the rigid ligamentum flavum. Therefore, it is commonly practiced that, when a Touhy needle is inserted into the spinal column, the operator identifies when the needle tip enters the epidural space across the ligamentum flavum by sensing a loss of resistance coming from the needle after passing through the rigid ligamentum flavum where the needle receives resistance. Such operations require that the anesthetist be highly experienced and is capable of performing highly proficient techniques.
Then, there is a disclosure where an injection device is used to identify when a needle tip enters the epidural space. In the disclosure, a plunger is movably disposed in a syringe, and the plunger is being withdrawn toward the end of the syringe to bias a spring while the needle is inserted into the spinal column. When the needle tip enters the epidural space across the ligamentum flavum, the resistance of the ligamentum flavum is lost, and accordingly the plunger advances toward the front end of the syringe by the biased spring. Thereby, the entering of the needle tip into the epidural space is identified by the movement of the plunger (see Patent Document No. 1).
However, the injection device requires not only the special syringe and plunger but also the spring. These requirements increase the number of components and makes the structure complex.
Another disclosure shows a device that identifies when a needle tip enters into the epidural space. The device includes an air inlet opening where air inflows from a syringe, an air output opening where the air outflows into the needle in which the air flow is into the epidural space, and a membrane (diaphragm, gasket) that bulges by a pressurized air flow from the air inlet opening while the air supply opening is closed. And after the needle is inserted into the back of the patient or into the ligamentum flavum where the resistance is given to the needle, the pressurized air is supplied from the syringe to bulge the membrane. Then the needle is inserted farther toward a spinal column, and when the needle tip enters into the epidural space, the bulged membrane shrinks and flattens as the pressurized air in the membrane flows into the epidural space (see Patent Document 2).